A cellular radio telephone network is made up of Base Transceiver Stations (BTS) each serving a corresponding, geographical “cell” area. Groups of several adjacent cells are organised into what are known as “Location Areas” (LA). If the MS is actively communicating with the network, e.g. the user is conducting a telephone call, the actual cell within the LA in which the MS is present is known to the network. However, when the MS is switched on but not actively used (i.e. it is IDLE or “camping”), the network only knows the LA but not the actual cell in which the MS is present. Cell sizes vary considerably and even if the network knows the cell in which the MS is present the position of the MS can only be identified with a resolution equal to the cell size.
Future cellular systems may be required to determine the position of a MS with a considerably better precision than can currently be obtained. For example, the United States Federal Communication Commission (FCC) has specified that when a MS is used to make an emergency call the network must be able to locate the MS with an accuracy of 125 meters in 67 percent of cases. Such precise position determination has many other desirable applications such as for taxi dispatch and for monitoring the whereabouts of vehicles, objects and persons etc.
One possible way to provide the necessary precision is to incorporate a satellite based positioning system, like the Global Positioning System (GPS ), into a MS. GPS can be used almost without geographical restrictions, but this solution is complex and increases the cost, size and power consumption of a MS. Moreover, GPS has additional problems including; low signal levels inside buildings, the difficulty in obtaining a clear path to at least three satellites in built-up urban areas, and the relatively long time to obtain a first GPS positional fix after switch-on.
It appears that a cellular MS locating system based on the cellular radio system offers the best and most practical solution. A number of proposals have been made based either on direction finding, field strength or time measurements. In particular, proposals have been made which rely upon determining the position of a MS from information transmitted to the MS from BTSs serving the cell in which the MS is present and/or adjacent cells.
One proposal includes triangulation of the position of a MS using at least two direction finding receivers within the network, e.g. incorporated into respective BTSs. Within built-up urban areas, multipath signals will degrade position precision unless the disturbing multipath distortion is removed from observed signals. However, good direction finding receivers having this capability are expensive and bulky and as such this method is unlikely to be suitable for large-scale cellular radio locating.
A second proposal relies upon measuring the relative field strengths of signals received at a MS from at least three BTSs. However, it will be appreciated that field strength is likely to vary considerable and in an unpredictable manner over the geographical coverage area of a BTS. This renders the method unsuitable in practise for cellular radio positioning.
A third proposal requires measurement of time delays in signals transmitted to a MS from several adjacent BTSs (or vice versa). Measured delays are converted into respective distances and a simple circle intersecting method can be used to determine a MS position. Such a position determining method, adapted for the GSM cellular radio system, is described for example in WO 92/05672 and WO 97/27711. The methods makes use of transmission Time Advance (TA) values already calculated in GSM transmissions in order to ensure synchronisation of MSs to BTSs (i.e. so that transmissions from a MS arrive at a BTS in the right time slot regardless of the distance between the MS and the BTS). FIG. 1 illustrates a TA value based position determining method.
FIG. 2 illustrates another time delay based positioning system in which Observed Time Differences (OTD) are utilised. EP0767594 describes such a system adapted for the GSM cellular radio system. The system has the advantage that OTDs can be obtained without having to register the MS with all of the BTSs used for position determination, as is the case with the TA value based systems.
Time delay based systems provide a cost-effective and simple solution to the problem of providing high accuracy position determination. However, accuracy is critically dependent upon the position of the BTSs relative to a MS whose position is to be determined. FIG. 3a illustrates an example of a good measurement geometry, where each pair of BTSs locates a MS along a hyperbola, and the set of hyperbolas intersect with large contact angles. In contrast, FIG. 3b illustrates an example of a bad measurement geometry where the hyperbolas intersect with small contact angles.